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At least 757 records · Page 42Linked to original sources
USGS2010

Significance of Klamath rocks between the Franciscan Complex and Coast Range ophiolite, northern California

Small fault‐bounded slabs of low‐grade (prehnite‐pumpellyite‐bearing) slate, metagraywacke, and greenstone occur between the Coast Range ophiolite and South Fork Mountain Schist for at least 60 km south of the Klamath Mountains, northern California. The metagraywacke slabs differ from typical Franciscan Complex metagraywacke to the west by the absence of blueschist‐facies minerals and the abundance of quartz and chert clasts, and they differ from sandstone and mudstone of the Great Valley sequence to the east by the presence of a penetrative cleavage and low‐grade metamorphism. The metasedimentary rock in the slabs is petrographically identical to the Jurassic Galice Formation, which occurs along the western and southern margins of the Klamath Mountains. Upward drag along a west dipping reverse fault best explains the present structural position of the slabs and is consistent with available geological and geophysical data. These data imply that the Coast Range ophiolite structurally overlies Sierran‐Klamath basement at least locally.

Oregon, Washington↗
USGS2010

Attenuation of the Coast Range ophiolite by extensional faulting and nature of the Coast Range "thrust," California

The late Mesozoic Coast Range ophiolite and Great Valley sequence in California were juxtaposed against the Franciscan Complex during a long tectonic history that included imbricate thrust faulting, low‐angle detachment, and high‐angle reverse faulting. Many low‐angle faults previously mapped as thrusts invariably juxtapose younger over older rocks, suggesting a normal sense of offset. We infer that serpentinite melange that is present structurally beneath the Coast Range ophiolite formed above the subduction zone during convergence and was subsequently faulted and further attenuated with upper plate rocks concurrent with extension. Franciscan blueschist‐facies rock is inferred to have been transported from depth to higher structural levels concurrent with underplating and extensional unroofing in the upper plate. The present juxta‐position of the Coast Range ophiolite and Great Valley sequence with Franciscan rocks is commonly controlled by Neogene high‐angle faults. We propose that the term Coast Range thrust is no longer appropriate and that the name should be changed to Coast Range fault.

California↗
USGS2014

Igneous activity in the Comstock District, Nevada

The oldest igneous rocks in the Comstock District are amphibolites probably derived from basalts and of Triassic age. These are intruded by pre‐Tertiary quartz monzonlte and by granodiorite of Sierran facies, the latter not being exposed on the surface but found on mine‐dumps. Igneous activity recorded mainly in volcanic rocks was almost continuous throughout the Tertiary. Its products, in order of age, were as follows: Eocene—Rhyolitic flows aggregating a few hundred feet in thickness: One intrusive plug of rhyolite occurs. Eocene or Miocene—Hornblende‐auglte andesite intrusive into the rhyolite, type locality American Ravine. ©1938. American Geophysical Union. All Rights Reserved.

Nevada↗
USGS2014

Petrology and structure of the Moa Chromite district, Oriente Province, Cuba

Chromite deposits In the Moa district, on the north coast of Oriente Province, Cuba, form one facies of the ultramafic complex in which they occur. Two planar structures, one a compositional layering due to variations in the relative proportions of olivine, pyroxene, plagioclase feldspar, and chromite, and the other a foliation caused by the orientation of pyroxene crystals in peridotite, are in general parallel, but diverge in a few places. Dips are fairly constant, being moderate to the northwest over most of the area, but sharp reversals occur immediately adjacent to irregularities in the chromite deposits. Two prominent joint sets are present: (1) Cross joints striking parallel to the strike of the foliation and dipping approximately at right angles to it; and (2) longitudinal joints striking parallel to the direction of dip of the foliation and nearly vertical. Gabbro dikes follow the first set, chrysotlle veins the second. Within the ore bodies dikes are commonly coarser‐grained, larger, and far more abundant than in peridotite; they enclose coarse, angular fragments of chromite. ©1947. American Geophysical Union. All Rights Reserved.

Eos, Transactions, American Geophysical Union↗
USGS1988

Granulite fades Nd-isotopic homogenization in the Lewisian complex of northwest Scotland

A published Sm–Nd whole-rock isochron of 2,920 ± 50 Myr, obtained from a wide range of lithologies in the Lewisian complex of north-west Scotland, was interpreted 1 as the time of protolith formation. This date is ∼ 260 Myr older than estimates for the timing of high-grade metamorphism in the complex at ∼ 2,660 Myr 2'3 , and this period is considered to represent the duration of the Lewisian crustal accretion-differentiation superevent (CADS) 4 . Here we give new Sm-Nd data, obtained specifically from granulite facies tonalitic gneisses, that yield a date of 2,600 ±155 Myr. Although depleted-mantle model ages ( t DM suggest >200 Myr of premetamorphic crustal residence, the regression date and its associated initial Nd-isotopic parameters demonstrate Nd-isotopic homogenization during the high-grade event, as well as the probability of general rare-earth-element (REE) mobility. Models for selective element depletion in the complex have previously assumed REE immobility since 2,920 Myr, but the data presented here suggest that a reappraisal of the depletion mechanism is required.

Nature↗
USGS1988

Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian/Turonian boundary

Perhaps the most significant event in the Cretaceous record of the carbon isotope composition of carbonate 1,2 , other than the 1–2.5 ‰ negative shift in the carbon isotope composition of calcareous plankton at the Cretaceous/Tertiary boundary 3 , is the rapid global positive excursion of ∼ 2 ‰ ( 13 C enrichment) which took place between ∼ 91.5 Myr and 90.3 Myr (late Cenomanian to earliest Turonian (C/T boundary event)) 1,4,5 . This excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon (TDC) reservoir resulting from an increase in rate of burial of 13 C-depleted organic carbon, which coincided with a major global rise in sea level 5 during the so-called C/T oceanic anoxic event (OAE) 6 . Here we present new data, from nine localities, which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the C/T boundary 7,8 is nearly synchronous with that for carbonate and is widespread throughout the Tethys and Atlantic basins (Fig. 1), as well as in more high-latitude epicontinental seas. The postulated increase in the rate of burial of organic carbon may have had a significant effect on CO 2 and O 2 concentrations in the oceans and atmosphere, and consequent effects on global climate and sedimentary facies.

Nature↗
USGS2003

Impact of structural and autocyclic basin-floor topography on the depositional evolution of the deep-water Valparaiso forearc basin, central Chile

The Valparaiso Basin constitutes a unique and prominent deep-water forearc basin underlying a 40-km by 60-km mid-slope terrace at 2.5-km water depth on the central Chile margin. Seismic-reflection data, collected as part of the CONDOR investigation, image a 3-3.5-km thick sediment succession that fills a smoothly sagged, margin-parallel, elongated trough at the base of the upper slope. In response to underthrusting of the Juan Ferna??ndez Ridge on the Nazca plate, the basin fill is increasingly deformed in the seaward direction above seaward-vergent outer forearc compressional highs. Syn-depositional growth of a large, margin-parallel monoclinal high in conjunction with sagging of the inner trough of the basin created stratal geometries similar to those observed in forearc basins bordered by large accretionary prisms. Margin-parallel compressional ridges diverted turbidity currents along the basin axis and exerted a direct control on sediment depositional processes. As structural depressions became buried, transverse input from point sources on the adjacent upper slope formed complex fan systems with sediment waves characterising the overbank environment, common on many Pleistocene turbidite systems. Mass failure as a result of local topographic inversion formed a prominent mass-flow deposit, and ultimately resulted in canyon formation and hence a new focused point source feeding the basin. The Valparaiso Basin is presently filled to the spill point of the outer forearc highs, causing headward erosion of incipient canyons into the basin fill and allowing bypass of sediment to the Chile Trench. Age estimates that are constrained by subduction-related syn-depositional deformation of the upper 700-800m of the basin fill suggest that glacio-eustatic sea-level lowstands, in conjunction with accelerated denudation rates, within the past 350 ka may have contributed to the increase in simultaneously active point sources along the upper slope as well as an increased complexity of proximal depositional facies.

Basin Research↗
USGS2002

Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U-Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U-Pb apatite ages are 2.4-2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz-biotite-plagioclase-garnet-staurolite-kyanite in the pelitic schists; (b) quartz-biotite-plagioclase-low-Ca amphiboles-kyanite in Mg-Al-rich schists, and locally (c) hornblende-plagioclase-garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg-Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from > 7 kbar to < 3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78-1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.

Journal of Metamorphic Geology↗
USGS2003

Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

The Solund-Hyllestad-Lavik area affords an excellent opportunity to understand the ultrahigh-pressure Scandian orogeny because it contains a near-complete record of ophiolite emplacement, high-pressure metamorphism and large-scale extension. In this area, the Upper Allochthon was intruded by the c. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410-400 Ma. Within < 5-10 Myr, all these units were exhumed by the Nordfjord-Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near-ultrahigh-pressure rocks to within c. 3 km vertical distance from the low-grade Solund Conglomerate.

Journal of Metamorphic Geology↗
USGS2016

Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina, USA

Metaultramafic rocks (MUR) in the Ashe Metamorphic Suite (AMS) of northwestern North Carolina include quartz ± feldspar-bearing QF-amphibolites and quartz-deficient, locally talc-, chlorite-, and/or Mg-amphibole-bearing TC-amphibolites. Some workers divide TC-amphibolites into Todd and Edmonds types, based on mineral and geochemical differences, and we provisionally add a third type – olivine ± pyroxene-rich, Rich Mountain-type rocks. Regionally, MUR bodies range from equant, Rich Mountain- to highly elongate, Todd-TC-amphibolite-type bodies. The MURs exhibit three to five mineral associations containing assemblages with olivine, anthophyllitic amphibole, Mg-hornblende, Mg-actinolite, cummingtonite, and serpentine representing decreasing eclogite to greenschist facies grades of metamorphism over time. MUR protoliths are difficult to determine. Southwestern MUR bodies have remnant olivine ± pyroxene-rich assemblages representing ultrabasic-basic, dunite-peridotite-pyroxenite protoliths. Northeastern TC-amphibolite MURs contain hornblende and actinolitic amphiboles plus chlorites – aluminous and calcic assemblages suggesting to some that metasomatism of basic, QF-amphibolites yields all TC-amphibolites. Yet MgO-CaO-Al 2 O 3 and trace element chemistries of many TC-amphibolites resemble compositions of plagioclase peridotites. We show that a few AMS TC-amphibolites had basaltic/gabbroic protoliths, while presenting arguments opposing application of the metasomatic hypothesis to all TC-amphibolites. We establish that MUR bodies are petrologically heterolithic and that TC-amphibolites are in contact with many rock types; that those with high Cr, Ni, and Mg have olivine- or pyroxene-dominated protoliths; that most exhibit three or more metamorphic mineral associations; and that contacts thought to be metasomatic are structural. Clearly, different MUR bodies have different chemistries representing various protoliths, and have different mineral assemblages, reflecting both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.

North Carolina↗
USGS2010

Structure and metamorphism of the Franciscan Complex, Mt. Hamilton area, Northern California

Truncation of metamorphic isograds and fold axes within coherent terranes of Franciscan metagraywacke by intervening zones of melange indicate that the melange is tectonic and formed after the subduction-related metamorphism and folding. These relations are expressed in two terranes of blueschist-facies rocks of the Franciscan Complex in the Mt. Hamilton area, northern California-the Jurassic Yolla Bolly terrane and the structurally underlying Cretaceous Burnt Hills terrane. Local preservation in both terranes of basal radiolarian chert and oceanic basalt beneath continent-derived metagraywacke and argillite demonstrates thrust repetition within the coherent terranes, although these relations are scarce near Mt. Hamilton. The metagraywackes range from albite-pumpellyite blueschists to those containing well-crystallized jadeitic pyroxene, and a jadeite-in isograd can be defined in parts of the area. Primary bedding defines locally coherent structural orientations and folds within the metagraywacke units. These units are crosscut by thin zones of tectonic melange containing blocks of high-grade blueschist, serpentinite, and other exotic rocks, and a broader, but otherwise identical melange zone marks the discordant boundary between the two terranes.

California↗
USGS1991

Zoned garnets in metapelites in western Fiordland, southwest New Zealand: polychronic crystallisation and insight into the nature and extent of early Cretaceous regional metamorphism

Two distinct types of compositional zoning are recognised in idioblastic to subidioblastic garnets in amphibolite facies metapelites in western Fiordland. One type occurs in kyanite (?? sillimanite)-bearing rocks and is characterised by a core to rim increase in CaO content, and a sympathetic decrease in FeO and MgO, with MnO remaining essentially constant, or decreasing slightly near the rim. This zoning pattern formed during garnet growth, and records adjustment of mineral equilibria under conditions of increasing load pressure. A much less common type of zoned garnet occurs in sillimanite + K-feldspar-bearing metapelites and is characterised by a core to rim decrease in CaO and MgO contents and a sympathetic increase in FeO and MnO near the rim. This zoning pattern is interpreted to result from post-growth volume diffusion, and records metamorphism at significantly lower pressures. -from Author

New Zealand Journal of Geology and Geophysics↗
USGS1991

The Briggs Creek Amphibolite, Klamath Mountains, Oregon: Its origin and dispersal

The Briggs Creek Amphibolite (BCA) in southwestern Oregon is an allochthonous slice of metamorphosed oceanic crust emplaced during the Nevadan orogenic event. Even though the evidence is not clear, the authors prefer to have the protolith for the BCA formed in a back‐arc basin contemporaneously as part of the Josephine Ophiolite during late Callovian to early Oxfordian. Although major rearrangement of some of the chemical elements took place during metamorphism, it is possible, by use of major and rare earth elements, to suggest that the BCA formed in an oceanic spreading centre rather than an island arc setting. Meta‐morphic mineral assemblages and composition of the hornblendes indicate that metamorphism attained amphibolite facies grade. Widespread occurrence of allochthonous blocks of BCA in Western Oregon indicate that its dispersion following metamorphism can be related to the accretionary history of the continental margin. Westward thrusting of these blocks of BCA over the Chetco Arc during the Nevadan orogenic event provided blocks that slid into the outboard active Early Cretaceous trench. These blocks were then incorporated into the downgoing subduction complex and overprinted by high P/T metamorphic assemblages. Tectonic blocks within the Early Cretaceous Otter Point trench melange (Franciscan Complex equivalent) contain amphibolites correlated with the BCA and overprinted by high P/T mineral assemblages.

Oregon↗
USGS1994

Physical geology and eruptive history of the Matahina Ignimbrite, Taupo Volcanic Zone, North Island, New Zealand

The Matahina Ignimbrite is a 280 ka ash‐flow sheet that erupted from Haroharo Caldera in the Okataina Volcanic Centre, northern Taupo Volcanic Zone, North Island, New Zealand. The ignimbrite underlies a 2000 km 2 area mainly east of the caldera, ranges in thickness from 5 to 200 m, and has a outflow volume of c. 120 km 3 , equivalent to c. 75 km 3 of magma. It is a multiple‐flow, compound cooling unit consisting of a basal tephra (fallout) member and three ash‐flow members, designated lower, middle , and upper , that record three eruptive pulses separated by brief time intervals, estimated from cooling and compaction noddling to range from 20 to 60 days. Distribution of coarse lithic clasts, together with local interbedded co‐ignimbrite lag breccias and tephra layers east of the Puhipuhi Easin, confirm Haroharo Caldera as the eruptive source. Over most of its extent on the Kaingaroa Plateau, the outflow s leet thickens eastward away from its source and attains its greatest thickness in the elongate, north‐trending, fault‐a ngle trough formed between the gently east sloping surface of the plateau and the western front of the Ikawhenua Fange. Difference in thickness of the ignimbrite across the fault bounding the west front of the Ikawhenua Range suggests that during the short time interval between emplacement of the lower and middle ash‐flow members, a major tectonic event caused at least 10 m displacement on the fault locally. Circumstantial evidence supporting this early syneruptive tectonic event is found along the Bay of Plenty coast where penecontemporaneous liquefaction structures, possibly seismically induced, occur in the distal subaqueous facies of the lower ash‐flow member. The presence of such 1 quefaction structures in the Matahina and other coastal New Zealand ignimbrites suggests a possible close association between tectonism and major ignimbrite eruptions.

North Island, Taupo Volcanic Zone↗
USGS2017

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.

New Zealand Journal of Geology and Geophysics↗
USGS1996

Trace fossils from Jurassic lacustrine turbidites of the Anyao Formation (Central China) and their environmental and evolutionary significance

The Lower Jurassic Anyao Formation crops out near Jiyuan city, western Henan Province, central China. It is part of the infill of the nonmarine early Mesozoic Jiyuan‐Yima Basin. In the Jiyuan section, this unit is about 100 m thick and consists of laterally persistent, thin and thick‐bedded turbidite sandstones and mudstones displaying complete and base‐or top‐absent Bouma sequences, and thick‐bedded massive sandstones. The Anyao Formation records sedimentation within a lacustrine turbidite system developed in a pull‐apart basin. Processes involved include high and low density turbidity currents, sometimes affected by liquefaction or fluidization. Facies analysis suggests that this succession is formed by stacked aggradational turbidite lobes. The absence of thick mudstone packages indicates that background sedimentation was subordinate to high frequency turbidite deposition. The Anyao Formation hosts a moderately diverse ichnofauna preserved as hypichnial casts on the soles of thin‐bedded turbidite sandstones. The ichnofauna consists of Cochlichnus anguineus, Hel‐minthoidichnites tenuis, Helminthopsis abeli, H. hieroglyphica, Mono‐morphichnus lineatus, Paracanthorhaphe togwunia, Tuberculichnus vagans, Vagorichnus anyao, tiny grazing trails, and irregularly branching burrows. Vagorichnus anyao occurs not only as a discrete trace, but also as a compound ichnotaxon intergrading with Gordia marina and Tuberculichnus vagans. Both predepositional and post‐depositional traces are present on the soles of turbidites. This ichnofauna comprises both feeding and grazing traces produced by a deposit‐feeding lacustrine benthic biota. Crawling traces are rare. Although certain ichnofossils (e.g. V. anyao, P. togwunia) show overall similarities with deep‐sea agrichnia, they differ in reflecting remarkably less specialized feeding strategies, displaying overcrossing between specimens (and to a lesser extent, self‐crossing), and in the case of V’ anyao recording post‐turbidite burrowing activity. The development of less specialized strategies than those displayed by deep‐marine ichnofaunas may be related to less stable conditions, typical of lake settings. Oxyenation, energy, sedimentation rate (both event and background), food supply, soft‐sediment deformation and erosion rate have mainly influenced trace‐fossil distribution. Turbidity currents would have ensured oxygen (as well as food) supply to deep lake settings, thus allowing the establishment of a moderately diverse biota. Biogenic structures were mostly confined to the outer, low energy areas. High sedimentation rates and strong erosion precluded preservation of ichnofossils in inner lobe settings. The Anyao ichnofauna is of significance in furthering knowledge of the colonization of deep lakes throughout the stratigraphie record and in identification of additional nonmarine ichnofacies. The ana‐lyzed ichnofauna resembles late Paleozoic lacustrine assemblages described from different localities around the world and is regarded as a Mesozoic example of the Mermia ichnofacies. However, when compared with Paleozoic assemblages, the Anyao ichnofauna shows a clear dominance of burrows over surface trails, deeper burrowing penetration, larger size, and presence of relatively more complex structures. The high burrow/surface trail ratio may be indicative of lower preservation potential in the latter, thus reflecting a tap‐honomic overprint. In contrast to the Paleozoic examples, the establishment of a relatively well‐developed lacustrine infauna may have precluded preservation of surface trails. Burrower activity probably obliterated biogenic structures formed close to the sediment‐water interface.

Ichnos: An International Journal for Plant and Ani↗
USGS2013

Petroleum fingerprinting with organic markers

Petroleum fingerprinting is an invaluable tool in forensic geochemistry. This article summarizes applications of fingerprinting in several oil spills and natural oil seepages that we have studied during the last 25 years. It shows how each unique chemical fingerprint can be used to correlate or differentiate oils. Fingerprints can provide information about processes in the environment that impact oils such as weathering and microbial degradation. They can be used to evaluate organic matter that contributed to oils, and classify oils with regard to the geological framework of their source, such as evaluating geological facies, age, lithology, and depositional environment.

Environmental Forensics↗
USGS2005

Paleoproterozoic metamorphism in the northern Wyoming province: Implications for the assembly of Laurentia

U-Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high-grade metamorphism at ???1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ???3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (<50 ??m) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well-defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ???100 m.yr. younger than the proposed time of Wyoming-Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ???1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia. ?? 2005 by The University of Chicago. All rights reserved.

Journal of Geology↗